Abstract

The spray form development from a state of the art multi-hole injector for gasoline
direct injection internal combustion engines is examined to attempt to determine the
thermo-fluid dynamics affecting the spray development. The current state of knowledge
regarding spray break-up and the interactivity of the factors on spray form are detailed.
The spray under investigation was injected into purposely designed quiescent chambers
to decouple the effects of the fluid mechanics on spray development from any in-engine
effects. The pressure chambers, experimental apparatus and techniques used to
characterise and measure the spray properties are described along with an assessment of
any sources of variability in the measurement and analysis methodologies and
hardware. Initial spray images of the spray produced by a range of multi-component
“retail” fuels as well as single component non-oxygenated and oxygenated
hydrocarbons with a range of boiling ranges and points for different injector body (and
hence assumed fuel) temperatures and chamber gas pressures are presented. The
experimental measurements show the strong interaction between the operational
conditions in relation to the fuel properties and the physical spray form. A large amount
of deviation from the nominal “ambient” spray form is observed for conditions where
the fuel’s bubble point (boiling temperature at given gas pressure) is exceeded by a
multiple of 10, termed spray collapse. The dependence of a multi-component fuel on the
boiling characteristics of its highest volatility components suggest that it is these
components which drive the fuel spray development formation, which is further
illustrated by comparing different single component fluids. This suggests that higher
volatility fluids are better representatives of full range, multi-component fuels for
modelling or other investigative work when a single component fuel is required to be
used. The onset of spray collapse was found to be gradual with no sudden “threshold”
condition at which collapse occurred, also illustrated by a gradual reduction in measured
spray droplet size with increasing injector body temperature and/or reducing gas
pressure. The physical factors affecting spray development and break-up, and their
effects are examined including the fluid flow inside a real size transparent, optically
accessed nozzle, illustrating the effect of cavitation supplying nucleation sites for the
subsequent vaporisation of the fuel. The scales of local air turbulence are found to affect
the local vapour concentration, and hence vaporisation rate, and hence the interaction of
these factors is shown to determine the spray formation.

Type:

Thesis
(Doctoral)

Title:

Factors affecting the development of sprays produced by multihole injectors for direct-injection engine applications